This invention relates to hearing aids, and more particularly to a hearing aid the output characteristic of which is automatically controlled according to the ambient noise.
In a conventional hearing aid which operates according to environmental conditions, the input signal is divided into two frequency components by a low-pass filter and a high-pass filter, and only the low frequency component is suppressed by a compression circuit. With the hearing aid, the quality of sound of voices in a noisy environment is scarcely affected. However, as the low frequency component of the noise is merely suppressed, i.e., the level is merely suppressed, when the level of a voice to be heard is increased with the level of the noise, apparently the S/N ratio of the voice is improved; however, when only the noise level is increased with the sound level maintained unchanged, and S/N ratio is not improved.